1. Technical Field
The present invention relates to a method of fabricating a polysilicon thin film and to a thin film transistor using the polysilicon fabricated by the same method and, more particularly, to a method of fabricating a polysilicon thin film and to a thin film transistor using the polysilicon fabricated by the same method, whereby thermal treatment temperature and its time are reduced to prevent a substrate from bending and to cause the substrate to have less defects.
2. Related Art
An active organic electroluminescent display device includes a thin film transistor (TFT) used to apply current to a pixel region and a peripheral driving circuit region. The TFT commonly uses polysilicon. In general, the polysilicon is formed by crystallizing amorphous silicon.
Typical crystallization methods are mainly classified into a low-temperature crystallization method and a high-temperature crystallization method based on their crystallization temperature, that is, based on a temperature higher or lower than 500° C.
An excimer laser annealing (ELA) method using an excimer laser is commonly used as the low-temperature crystallization method, which may use a glass substrate because the method is performed at a crystallization temperature of about 450° C. However, the method causes fabrication cost to increase and causes the optimal size of the substrate to be limited, which leads to an increase in total cost for fabrication of the display device.
The high-temperature crystallization method includes a solid phase crystallization method, a rapid thermal annealing (RTA) method, and the like, which methods are widely used as a low-cost crystallization method.
However, the solid phase crystallization method requires the crystallization to be performed for 20 hours or more at a temperature of at least 600° C., and this can cause crystallization defects to occur in the crystallized polysilicon so that sufficient mobility cannot be obtained, and so that the substrate is easily deformed during the thermal treatment process. In addition, the glass substrate cannot be used because of the high-temperature crystallization. When the crystallization temperature is lowered, productivity is reduced.
In the meantime, the RTA method may allow the process to be performed in a relatively short time. However, the RTA methods that have been developed up to the present have disadvantages in that the substrate is easily deformed due to severe thermal shock and the electrical properties of the crystallized polysilicon are not good.
Accordingly, the method that must be used is a high-temperature thermal treatment method which has low cost when the crystallization is carried out in order to reduce the fabrication cost of the active device. However, there is also a need for the development of a high-temperature thermal treatment method having superior crystallization properties, which method uses a glass substrate with low cost and does not cause a problem such as bending of the substrate.